A system of a vehicle applies an excitation wave to an exciting coil of a resolver. The system receives two signal waves from a sin phase coil and a cos phase coil of the resolver. The excitation wave has a sinusoidal waveform having a constant amplitude level. A signal wave is referred to as an amplitude-modulated wave having a waveform obtained by amplitude-modulating the excitation wave by a rotation angle. For example, two-phase signal waves are sin θ·sin ωt and cos θ·sin ωt. ω represents an angular frequency of the excitation wave or the excitation signal.
In prior art, there has been known a resolver digital convertor which AD (analog-to-digital) converts an AC signal output from a resolver and detects a point at which the amplitude of the AC signal is 0. There has been known a circuit which obtains a rotation angle by measuring a time difference of a zero-cross point at which voltage levels of two-phase AC signals cross the zero level.